Introduction

The Sigma AF 12-24mm f/4.5-5.6 EX DG HSM is another example of Sigma's recent quests to
push the limits of the current state of lens design to the max - it is the most extreme
full-frame ultra-wide zoom on the market today with no competition around. In fact there
isn't even a (mass production) rectilinear SLR fix-focal this wide. Quite an achievement!
As such it is also interesting for APS-C DSLR users where the lens has still a field-of-view
equivalent to 19-38mm on full-frame cameras so even here it remains an ultra-wide zoom.

The lens construction is made of 16 elements in 12 groups including 4 Special Low Dispersion
(SLD) elements and 3 aspherical elements - an indication for the extremely complex design.
Its aperture mechanism features 6 aperture blades.
When compared to e.g. the Canon EF-S 10-22mm f/3.5-4.5 USM the Canon lens is almost dwarfish - the
Sigma is a very fat lens at 87x103mm and despite its rather slow aperture range it weights a
hefty 600g.

Due to the extreme design Sigma had to choose a very protruding front element
so there's no front filter thread by default. However, when attaching the lens
cap holder there is a option to mount a filter here (82mm). At 12mm this can
cause slight edges shading though so front filters can only be used from about
~14mm & up (I wasn't able to test the limit though). There's a gelatine filter
slot at the rear of the lens (useful for color filters for B&W photography).

The lens has a pretty fast and near silent HSM (hypersonic motor) AF drive which
provides full-time manual focusing in one-shot AF mode. The minimal focus distance is
0.28m resulting in a max. magnification of 1:7.1 at 24mm.
The build quality of the lens is very impressive and among the best of what I've
seen from Sigma so far. The typical EX finish (smooth crinkle style) feels
very pleasant and the zoom and focus control rings are very smooth and well damped.